Everything the model uses to answer — the instructions, the conversation so far, the files you pasted — must fit in the context window, measured in tokens. Outside the window, the model has no memory: it isn't consulting some database of your past chats unless a tool puts that text back in.

Windows have grown from 4,000 tokens to a million-plus, but bigger isn't free: long contexts cost more, run slower, and models still attend unevenly to the middle of very long inputs. A lot of agent engineering — summarizing, chunking, RAG — is really context-window management.